1. Field of the Invention
The present invention relates to a method and apparatus for detecting defects in dielectric films. Particularly, the present invention relates to method and apparatus for detecting and measuring detects in a dielectric film which is provided on the surface of silicon substrate.
2. Related Art
There are a number of means which have conventionally proposed, based on various principles, to detect defects in dielectric film which is provided on the surface of a silicon substrate. Example of such means include (1) electrical methods using capacitors, (2) methods using chemical selective etching, (3) electrochemical methods such as bubble generation etc., (4) chemical methods such as Nomarski method, (5) microprobe analysis methods using X-rays or electron beams, and (6) profile observation by a mechanical probe, as reviewed in RCA Review, Vol. 34, pp. 656-690 (1973) and Solid State Technology, Vol. 17, pp. 35-42 (1974).
However, for evaluation of films (SiO.sub.2, SiN, SiON etc.) used in insulating semiconductor elements and gate oxide films in MOS transistors, methods capable of detecting defects under the application of electric field are preferable for the object of the films.
A first method meeting the above requirement includes, for example, a method of measuring the electrical characteristics of MOS capacitors having a structure "metal (M)--oxide film (O)--semiconductor (S)", such as in the above method (1). In this method, the electrical characteristics of the dielectric film is directly measured to determine the presence or absence of defects and is thus a method of evaluating defects with extremely high reliability. However, construction of such MOS capacitors needs various steps requiring vacuum such as sputtering, vapor deposition and low-pressure CVD and steps including photographic etching step etc. Accordingly, there is the drawback that the rapidness and easiness of evaluation are lost.
To overcome the drawback of the first method, electrochemical methods which are capable of completing evaluation in a few steps are very effective, and various methods have conventionally been proposed to realize it. For example, in RCA Review, Vol. 31, pp. 431-438 (1970), a method of depositing copper on defects in a semiconductor silicon substrate having an dielectric film as a cathode by the phenomenon of electrophoresis, using a copper anode in methyl alcohol as solvent is proposed as a second method for overcoming the drawback of the first method. In this method, however, the specific electric conductance of methyl alcohol at 25.degree. C. is as very low as 3.times.10.sup.-7 .OMEGA..sup.-1 cm.sup.-1, and the influence of the voltage drop caused by the resistance of the solution is large, so the surface potential of the semiconductor silicon substrate having the dielectric film as the cathode is difficult to keep uniformly on the surface. Accordingly, uneven copper deposition in the surface occurs easily. Further, because of moisture absorption or evaporation of the solution, there are many problems such as easy changes with time in the liquid composition, poor reproducibility etc.
As a third method of overcoming the drawbacks of the first and second methods, Japanese Patent Appln. Laid-Open Publication No. (JPA) Sho 52-132,682 discloses a method of depositing copper above defects in a silicon semiconductor substrate having dielectric film as the cathode by electrochemical plating reaction, which comprises: immersing, in an electrolyte solution composed of aqueous solution containing strongly acidic salt of copper, an anode constituted of an electrically conductive substance not corrosive by said electrolyte solution and a cathode constituted of the semiconductor silicon substrate having dielectric film which is a material to be measured; and applying a direct current voltage across the anode and the cathode so that the applied direct voltage is smaller than the dielectric breakdown voltage of the dielectric film formed on the surface of the silicon substrate. In this method, the effect of the voltage drop due to the electrolyte solution is small, the surface potential of the silicon substrate can be kept uniform on the surface. Accordingly, the uniformity of copper deposition ability in the surface is good. Further, the electrolyte solution is an aqueous solution, so the change with time in the liquid composition caused by moisture absorption or evaporation is not a large problem.
In this method, the area of the copper deposit is varied depending on the size of the defect in the dielectric film, so the size of the defect in the dielectric film can be evaluated relatively by using the areas of the copper deposits on the surface of the dielectric film. In this case, it is possible to determine the central position of the defect by the central position of the deposited copper. If the area of deposited copper is so large that its diameter reaches 10 microns or more, the relative size and distribution of each individual defect in the film can be visually determined in terms of the relative size and distribution of its corresponding deposited copper on the surface.
However, the accuracy of specifying the central position of the defect is deteriorated with increase in area of deposited copper. Accordingly, if copper is deposited in such a size as to permit visual observation, it is difficult to accurately specify the central position of each individual defect for observing the defect. For processing a sample to be directly observed for its sectional defects, it is essential that the central positions of defects can be specified simultaneously and accurately, so the difficulty in specifying the central position of deposited copper is disadvantageous.
On the other hand, if the diameter of deposited copper is as small as 1 micron or less, particularly 0.1 micron or less, the central position of the defect can be accurately specified, but the visual observation of the relative size and distribution of each individual defect on the surface is difficult.
Further, a natural oxide film (silicon dioxide film) on the back of silicon substrate should be previously provided with ohmic electrode using aluminum or indium, making the procedure cumbersome.